This application seeks to achieve two broad objectives: (a) to employ laboratory-based procedures to investigate the acute effects of marijuana (i.e., -9 THC) on basic behavioral mechanisms in human subjects; and (b) to extend these methodological strategies in order to characterize potential residual effects in adolescents (15-18 years old) who use marijuana chronically. To that end, the specific aims of the application are detailed below. Marijuana use in the US has increased rapidly over the past decade, and is exceedingly prevalent among adolescents. Heavy marijuana use is associated with a number of clinically relevant outcomes, including learning and memory deficits, dropping out of school, and unemployment. The biological and behavior mechanisms that influence these detrimental outcomes (which effect both individuals and communities) are not yet well understood. Preliminary data from our laboratory and recent neuroscience studies suggest that basic reinforcement properties may be involved. Accordingly, the present studies will investigate marijuana effects on basic reinforcement processes, both acutely in adults and residually in chronic-using adolescents. Five studies are proposed, focused on marijuana effects on (a) sensitivity to rewards, (b) motivational processes, and (c) behavioral change and inhibition. Behavioral science has shown these variables to be important for adaptive behavior. The projects will measure acute marijuana effects in adult humans across three dose levels, and provide analogue measurement of adolescents with heavy marijuana use (measured by urinary cannabinoid levels and self-reports). The experiments will employ laboratory procedures that utilize choice paradigms with historically well-understood and clearly defined parameters. Collectively, these studies are designed to help elucidate the effects of marijuana on basic learning mechanisms, and the role of marijuana in the etiology of socially detrimental behavior.